This invention relates to a camera, and more particularly, to an exposure system for differentially varying the film exposure over a film frame.
In photographing a scene having large variations in spatial brightness, and particularly where exposure is carried out under the control of a photocell, it is not unusual for details in the brightest regions to be washed out by over-exposure and for details in the darkest region to be concealed by under-exposure of the photosensitive material on the film frame. For a given scene, the extent of the problem depends on the characteristic curve, (i.e., the density-log exposure curve) of the photosensitive material being used. Where the brightness range in the scene being photographed and the photosensitive film material are such that many details are lost by reason of the exposure being determined by the average scene brightness, improved results can be achieved, for some scenes, by controlling exposure in accordance with a spatially weighted average of scene brightness. Thus, it is conventional to weight the photocell viewing angle to reduce the response to light from the upper portion of a scene, which usually will be the sky during daylight, outdoor photography so that the photosensitive element will be more responsive to the central portion of the scene where a person's face, for example, is located in the usual photographic setting. In such case, the central region, where it is desired for the detail to be the most distinct, will dominate the photocell's response and will result in a more pleasing photograph at the expense of some regional over- or under-exposure in peripheral portions of the photograph. The photocell response can be modified by optics associated with the photocell. Alternatively, the response can be electronically modified such as shown in U.S. Pat. No. 3,409,378. Regardless of the technique utilized, however, any improved result will depend on whether the actual scene being photographed corresponds to the design criteria built into the photocell response.
Improvement in the recorded detail of the photographic image has been achieved by focal plane shutters having a modulated aperture width for differentially exposing individual regions of a film frame in accordance with the brightness of the regions in the scene being photographed. U.S. Pat. No. 3,116,670, which is typical of the above-noted shutter mechanism, discloses a focal plane shutter having means for controlling aperture width to modulate the light reaching the photosensitive material during the transit thereover of the shutter curtain. Such means includes a photocell positioned to receive light passing through the camera lens and reflected from a narrow strip at the leading edge of the curtain slot, and an electromagnet for modulating the width of the slot in accordance with the output of the photocell. Thus, the exposure of elemental strips of the photosensitive material is modulated in accordance with the average brightness of the scene within the strip. Other examples of variable focal plane shutters are described in U.S. Pat. Nos. 3,479,936 and 3,442,198.
While the above-noted shutter mechanisms may give improved results in scenes where the brightness gradient runs in the same direction as the direction of movement of the shutter, such improvements are generally limited when the brightness gradient in all or a part of the scene is parallel to the slot of the shutter curtain. Furthermore, by reason of the time required for a focal plane shutter to complete its traverse of a film frame, this approach is not suitable for a hand-held camera where maximum exposure time should not exceed about 1/30 of a second to prevent blurring due to reflex movements on the part of the photographer.
In the above-noted cameras, the exposure of selected scene areas are essentially compressed so that each area of the film frame receives approximately the same quantity of light flux and will have the same average density.
The value of the average density is dependent on the characteristic curve (density vs. logarithm exposure curve) of the photosensitive material. The predetermined amount of light that the shutter slot is permitted to transmit is preferably chosen to correspond to an exposure lying about midway in the linear portion of the characteristic curve. This choice maximizes the amount of detail that can be obtained, however, a photograph made in this manner, is not a realistic reproduction of a scene and if the compression is carried to the extreme, will be "flat" or "gray" in the sense of its being of uniform average density throughout. Since darker regions of a scene are expected by an observer to be much darker in a photograph than lighter regions in the scene, the resultant photograph will not in all cases be as aesthetically pleasing as is desirable.
It is therefore an object of the present invention to provide a new and improved exposure control system and apparatus for optimizing an exposure over the film frame.
Another object is to provide an improved exposure control system for selectively compressing the range of recorded scene brightness to enhance picture detail while retaining the relative brightness of many of the scene areas.